Titanium dioxide pigment

ABSTRACT

ELONGATE MONOCRYSTALLINE RUTILE NUCLEUS CRYSTALS ARE MIXED WITH AN ACIDIC TITANIUM CHLORIDE SOLUTION WHICH CONTAINS FROM 0.1 TO 60 GRAMS OF DISSOLVED TITANIUM PER LITER, FROM 0.004 TO 9 GRAM IONS OF CHLORIDE PER LITER AND LITTLE OR NO PHOSPHATE, SULPHATE OR BISULPHATE. THE MIXTURE IS HEATED AT A TEMPERATURE OF FROM 80* TO 150*C., THEREBY PRODUCING COMPOSITE TITANIUM DIOXIDE PIGMENT PARTICLES CONSISTING EACH OF ONE OR MORE NUCLEUS CRYSTALS SURROUNDED BY ELONGATE RUTILE CRYSTALLITES IN GENERALLY PARALLEL ARRAY TOGETHER WITH INTERSTITIAL NON-RUTILE TITANIUM DIOXIDE, THE PIGMENT PARTICLE HAVE ASPECT RATIOS OF FROM 3 TO 150, EFFECTIVE DIAMETERS OF FROM 0.05 TO 0.3 MICRON AND CONTAIN NOT MORE THAN ABOUT TEN WEIGHT PERCENT OF THE NON-RUTILE TITANIUM DIOXIDE.

United States Patent 3,749,764! TITANIUM DIOXKD'E PTGMENT Max G. Basque, Pittsburg, James E. Magnet, Antioch, Joseph P. Surls, Walnut Creek, and Bert F. West, Orinda, Calili, assignors to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Oct. 18, 1971, Ser. No. 189,827 Int. Cl. (301g 23/04 U.S. Cl. 423-610 5 Claims ABSTRACT OF THE DISCLOSURE Elongate monocrystalline rutile nucleus crystals are mixed with an acidic titanium chloride solution which contains from 0.1 to 60 grams of dissolved titanium per liter, from 0.004 to 9 gram ions of chloride per liter and little or no phosphate, sulphate or bisulphate. The mixture is heated at a temperature of from 80 to 150 C., thereby producing composite titanium dioxide pigment particles consisting each of one or more nucleus crystals surrounded by elongate rutile crystallites in generally parallel array together with interstitial non-rutile titanium dioxide. The pigment particles have aspect ratios of from 3 to 150, eifective diameters of from 0.05 to 0.3 micron and contain not more than about ten weight percent of the non-rutile titanium dioxide.

BACKGROUND OF THE INVENTION It is known to make rutile pigments by several dilferent processes involving hydrolysis of acidic titanium sulfate or chloride solutions in the presence of relatively minor proportions of colloidal seed suspensions. The hydrolysates obtained from sulfate solutions in'such processes consist of anatase-which has a lower refractive index than rutile and is thus a poorer pigmentand have either been used as such or have been upgraded to rutile by calcining.

U.S. Pat. 3,329,484 to Long et al. is directed to a hydrothermal process for the production of rutile pigments from acidic titanium chloride solutions such as iron-free leaches of ilmenite ores. By use of a colloidal rutile seed prepared in a specified manner and at hydrolysis temperatures of at least 130, hydrolysates consisting of acicular rutile monocrystals are taught to be obtained. These crystals range in diameter from 0.05 to 0.2 micron and have aspect ratios of from 3 to 15. They have utility as paint pigments exhibiting Reynolds tinting strengths of from 900 to about 2000. The maximum tinting strengths are shown by those crystals about 0.1 micron in diameter and having a length to diameter ratio of about 8.

At the temperatures (at least 175, preferably 210-260 C.) and chloride concentrations (about 8-10, preferably about 9) required for attainment of tinting strengths above 1350 (without calcining) by the process of the 484 patent, the hydrolysis mixture is very corrosive and exerts a considerable vapor pressure. To operate under these conditions necessitates the use of costly materials of construction and imposes high design pressures. The poorer quality pigments obtained under less severe conditions of corrosivity and stress can be upgraded to high tinting strengths by calcining. However, calcining is expensive.

U.S. Pat. 3,528,773 to Surls et al. teaches an alternative two-step hydrothermal route to high tinting strength rutile monocrytsals of the preceding type. In the first step a seeded titanium chloride solution which contains from 6 to 11 gram ions of chloride per liter is hydrolyzed at a temperature of 85-l50 C. The resulting hydrolysate particles are then mixed with a less volatile and/ or corrosive acid, such as nitric acid or with considerably more dilute (usually less 3,749,704 Patented July 31, 1973 SUMMARY OF THE INVENTION The invention resides in a composite titanium dioxide pigment particle of controlled size and shape for high efficiency of light scattering in disperse systems, and in the process by which the particle is made. It has been discovcred-that the particle of the instant invention can be made by a sequence of operations in which temperatures greater than 150 C. are not required in any step. The last operation in this sequence is a hydrolysis and broadly constitutes the process of the instant invention.

Elongate rutile monocrystals are contacted at 150 C. for at least 3 minutes with an acidic aqueous titanium chloride solution which contains little or no phosphate, sulphate or bisulphate. The monocrystals, individually or as clusters of several crystals, act as nuclei around which a major portion of the titanium dioxide hydrolysate deposits in the form of elongate rutile crystallites having their long axes generally parallel to the long (c) axes of the nuclei crystals, the balance of the hydrolysate depositing as non-rutile titanium dioxide between the nucleus crystals and/or the crystallites. The titanium chloride solution has a titanium content of from 0.1 to 60 grams per liter and a chloride content of from 0.004 to 9 gram ions per liter. The mixture is maintained within the temperature range of 80-150 C. until the particles present have grown to the desired average effective diameter. The weight of the solid phase at this point, w can be estimated as w =w (d /d where w is the weight of nuclei crystals charged, a is the average effective diameter desired for the product particles and d, is the average diameter of the nuclei crystals.

It is preferred to prepare the nuclei crystals at temperatures not in excess of C., but the invention can be practiced with nucleants made at higher temperatures and is not restricted to the use of nuclei crystals made under any one set of conditions or in any particular manner.

The composite titanium dioxide pigment particles prepared have a length within the range of from 0.15 to 15 microns.

The product particles of the invention have utility for both paint pigmentation and paper opacification.

DETAILED DESCRIPTION OF THE INVENTION Suitable nuclei crystals for the practice of the invention are rutile monocrystals from 0.15 to 15 microns long, from 0.01 to 0.1 micron in diameter and having aspect ratios of from 5 to 200 or more.

Because the ratio of end surface area to lateral surface area is relatively small for the nuclei crystals, accretion of hydrolysate results almost exclusively in cross-sectional growth of the developing particle. This is particularly so for nuclei of higher aspect ratios, in which case end growth is essentially nil. Consequently, the weight increase required to convert nuclei of a given diameter, d to product particles of a desired effective diameter, d is readily calculated from the fact (assuming one nucleus crystal per product particle) that the volume ratio of the product particle to the nucleant crystal is equal to the ratio of the squares of their respective diameters. That is,

uct, the Weight ratio may be substituted for the volume ratio. 'Ihus w /w =d /d and w =w (d /d where these terms are defined as above. In discussing the results of variations in reaction parameters in the working examples herein, the term growth ratio is used. This term refers to the ratio w /w The most preferred products made by the instant process are those of a relatively narrow size distribution and having the average effective diameter (about 0.1 micron) at which acicular rutile pigment particles are most efficient in light scattering. Accordingly, and for practical reasons of process efficiency, it is preferred to employ a nucleant having a maximum content of uniformly sized nucleus needles with diameters of about 0.01 to 0.04 micron (100 to 400 angstroms) and a minimum length of about 1 micron (10,000 angstroms).

For maximum nucleant utilization and product uniformity, the nucleus crystals are so well dispersed in the liquid phase that not more than one crystal is contained in each product particle. However, as a practical matter, complete separation of nucleus crystals is not required and no loss in product quality results if a substantial proportion of the product particles comprise several nucleus crystals each. Of course, the product particles will be somewhat reduced in number and larger in diameter at a given growth ratio when ideal dispersion of the nucleus crystals is not achieved. This can be countered by decreasing the growth ratio at which the hydrolysis is terminated by a small amount which is readily determined empirically. As a first approximation, the weight of solid phase at which the hydrolysis is to be terminated can be estimated from the preceding definition of growth ratio.

Preferably, the nucleant employed consists solely of the specified rutile monocrystals. However, minor amounts of impurities, such as anatase or brookite forms of TiO or of inert materials have little noticeable effect. Even nucleants comprising as much as 30 wt. percent of nonrutile TiO particles are operable. It is only necessary that the nucleant be so composed as to permit contacting of the elongate monocrystals present therein with the titanium chloride solution under the specified conditions of temperature and multivalent anion concentration.

The process of the invention can be carried out batchwise or as a continuous operation. In batch operation, the entire amounts of nucleant and titanium chloride solution are usually combined at once. In a semi-continuous mode of reaction, an initial mixture of nucleant and chloride solution is brought to temperature and then additional chloride solution is added at a rate such as to provide the desired concentration of dissolved titanium in the liquid phase. This usually amounts to adding dissolved titanium at a rate equal to the prevailing hydrolysis (and growth) rate, so that the concentration of dissolved titanium remains essentially constant. In a truly continuous embodiment, nucleant and titanium chloride solution can be metered to an agitated slurry of growing particles at the same rate at which the resulting mixture is withdrawn from the reactor. In this mode of operation, the percent of solids in the slurry is maintained at a sufficiently low level that the stirring input required to keep the reaction mixture essentially homogenous does not result in excessive shear and/or power demand.

Suitable titanium chloride solutions for the practice of the invention contain from 0.1 to 60 grams (0.002 to 1.25 g. atoms) of dissolved titanium, from 0.004 to 9 gram ions per liter of chloride, not more than 0.001 gram ions of phosphate per liter and not more than 0.01 gram ions of sulphate and/or bisulphate per liter.

Although the exact nature of the titanium chloride species present in the solutions employed in the practice of the invention is not certain, it is convenient to represent such solutions as consisting of water, TiOCl H+ and Cl- (the latter three presumably being hydrated). In stating the gram ions of chloride contained in the solution, the chloride content of the TiOCl is ordinarily counted as though complete hydrolysis had already occurred, according to the reaction If it is desired to estimate the hydrogen ion concentration in the liquid phase at any given stage of hydrolysis, the assumption can be made that the nominal concentration of H+ is equal to the concentration of chloride actually present as Cl (excluding that present as TiOCl That is, the hydrogen ion concentration is approximately equal to the gram atoms of chloride less twice the gram atoms of dissolved titanium (per liter). It will be seen, however, that it is not necessary to specify (H or pH (in addition to the titanium and chloride contents) in order to characterize the solution.

It will be recognized that, as here defined, the chloride content does not change as a result of hydrolysis.

The hydrolysis reaction is carried out, preferably with agitation, by heating the mixture of nucleant and titanium chloride solution to a temperature within the range of to 150 C. and thereafter maintaining the mixture within this temperature range until the weight of the solid phase therein corresponds to the desired growth ratio. The time required to secure the requisite weight increase with a given nucleant/chloride solution combination under a given set of conditions is readily determined by a series of several pilot hydrolyses terminated (as by rapid cooling) after successively longer contact times. The titanium chloride solution can be brought to the desired hydrolysis temperature before being mixed with the nucleant, but preferably this is accomplished rapidly and the nucleant is introduced as soon as possible after the solution reaches a temperature of 80 C. It is also feasible to add more titanium chloride solution as the hydrolysis proceeds. The hydrolysis is carried out in a reactor having an acid resistant interior surface and, preferably, fitted with means-such as a rocker, stirrer or slurry circulation pumpfor agitation of the reactor contents.

If the hydrolysis is carried out at a temperature (greater than about C.) such that significant losses of Water or acid could occur by volatilization, the reactor is sealed and the reaction is run under at least autogenous pressure. Preferably, the reactor is provided with tem perature regulation means which permit rapid heating or cooling of the contents. Suitable reactor lining materials are high silica glass or tantalum metal, for example.

When the desired growth ratio has been reached, the reaction mixture is cooled and the product particles separated, as by filtering, centrifuging or decanting. The

product is washed with water and may be used directly as a wet cake or dried.

It is important to select a combination of reaction temperature, chloride concentration and nucleant concentration in the reaction slurry such that an excessive growth rate does not result. At high growth rates, the hydrolysate tends to accrete as spikes or branches on the nucleus crystals, rather than in the essentially parallel disposition required for uniformity of particle diameter and high efficiency of light scattering.

Hydrolysis temperature At temperatures below 80 C., the rate of particle growth is impracticably low. At temperatures above C., the growth rate is too high. Also, the corrosiveness and vapor pressure of the system increases rapidly above 15 0 C. It is highly preferred to operate within the range of 90l10 C., where growth rates are adequately high and operating pressures of from 0 to a few p.s.i. (gauge) sufiice.

Chloride concentration The minimum operable chloride concentration is related to the dissolved titanium concentration, since the smallest amount of chloride associated with the titanium corresponds to the formula TiOCI Thus, at a minimal titanium content of 0.1 gram per liter, the chloride consolved titanium per liter is maintained at 100150 C. under at least autogenous pressure for 1 to 2 hours in admixture with a colloidal hydrous titanium dioxide seed dispersion (250% level; prepared, for example, as described in the 484 patent) and the resulting polycrystalline TiO hydrolysate is then contacted at 200-250 C. under at least autogenous pressure for from A to 4 hours with one of the following: 0.01 to 5.5 M aq. HCl, 0.01 to 20 M aq. HNO or 0.01 to 20 M HC By this method there are producedunder minimally corrosive conditions-rutile monocrystals of relatively low aspect ratio and small diameters which are suitable for use as nucleus crystals for the preparation of the composite pigment particles of the instant invention. This alternative route to relatively low aspect ratio nucleus crystals is part of the instant invention.

More preferred as nucleants are needleform rutile monocrystals claimed in and prepared according to US. patent application Ser. No. 167,786, filed July 29, 1971 in the names of D. L. Bauer, J. P. Surls and B. P. West as inventors. An acidic titanium chloride derived from a pure source material, such as TiCl and having a high (9.3-13 gram ions/liter) chloride content is hydrolyzed at a temperature of 120-325 C. in the presence of a rutile seed. The seed is either a colloidal dispersion of a type such as that described in the 484 patent or is a monocrystalline rutile needle product obtained by a previous such hydrolysis with a colloidal seed. The titanium chloride solution comprises from 20 to 120 grams of titanium per liter and the seed is employed in an amount sufficient to provide an additional amount of titanium of from 0.5 to 25 wt. percent of the dissolved titanium. The product needles obtained by this process have aspect ratios of from 20 to 200 and diameters of from 0.01 to 0.5 micron.

The hydrolysates produced by the latter process at temperatures below about 220 comprise minor amounts of non-rutile particles. For example, at a temperature of about 150", a dissolved titanium content of about 40 grams per liter, a chloride concentration of about 9.5 g. ions per liter and a seed level of about 2% the latter process produces a TiO hydrolysate consisting predominantly of needleform rutile crystals, the balance being peptizeable brookite and anatase particles. Such hydrolysates can be employed directly as nucleants in the instant process, but preferably the brookite and anatase particles are first largely removed, as by peptization with dilute acid and filtration.

It is thus possible to produce the instant composite pigment particles in an integrated sequence of operations, such as the following, without exceeding a temperature of 150 in any stage:

(1) TiCl is reacted with water at a temperature of 50-70 C. and HCl removed as necessary to provide a TiOCl concentrate comprising about 200 grams per liter of dissolved titanium and from 2 to 3 (preferably 2) gram ions of chloride per gram ion of titanium.

(2) A portion of concentrate is diluted with water to a titanium content of from 4 to 30 (preferably 8-15) grams per liter and agitated without appreciable backmixing at a temperature of 70150 (preferably about 80-100) for a period-depending on the temperature-- of from 60 to 0.1 minutes, preferably 30 to 2 minutes. Heating to temperature (or subsequent cooling, if the seed is not to be used immediately) is accomplished in a period of not over 20 minutes and preferably in 1 to 2 minutes. The resulting seed is a pearly, opalescent suspension of colloidal rutile particles.

(3) An acidic titanium chloride solution containing from 2-80 (preferably 20-50) grams of titanium and 9.3- 13 (preferably 9.510) gram ions of chloride per liter is provided, as by dilution of concentrate with hydrochloric acid of appropriate concentration or by mixing TiCl and hydrochloric acid. Seed from the preceding step is added in the amount of 0.1 to 50 (preferably 4-10) wt. percent of the dissolved titanium content of the solution and 8 the mixture maintained at 125-200 C. (preferably not over 150) for 0.5 to (preferably 1-4) hours.

(4) If the resulting hydrolysate comprises any nonrutile particles, these may be (preferably, are) largely removed by peptization and filtration.

(5) The hydrolysis of the instant invention is carried out as above described, employing nucleus crystals formed in the preceding step, and the composite product particles are recovered.

The important considerations in carrying out foregoing Steps 2 and 3 are effects of the reaction variables in both steps on the amount and quality of the product of Step 3.

In Step 2, shorter residence times at temperature and lower temperatures tend to enhance the formation (in Step 3) of rutile over non-rutile Ti0 but also tend to lower the yield of solids (hydrolysate). Shorter residence times also tend to result in greater rutile diameters (diameter in plane determined by X-ray diffraction) for the hydrolysate particles. Preparation of the seed from more dilute TiOCl solutions (lower dissolved titanium concentrations) is essential to high yields of hydrolysate (in Step 3) and results in higher rutile contents in the hydrolysates.

A good compromise for Step 2 is a holding time of about 10 minutes at 85 C. (or about 5 minutes at 100 C.) with a titanium concentration of 10-15 grams per liter and a chloride to titanium ratio of about 2. Seed prepared under these conditions gives a high yield of high rutile content hydrolysate in Step 3.

In Step 3, higher yields of hydrolysate and higher rutile contents result at higher dissolved titanium concentrations but the rutile diameter tends to fall off. A minimum of about 9 to 9.5 g. ions of chloride per liter is required (at C.) to get dispersed, monocrystalline rutile needles Which can be separated from the non-rutile fines. Increasing the chloride concentration beyond this point leads to lower hydrolysate yields. Higher seed levels generally tend to effect an increase in yield (but the seed dispersion must itself be more concentrated if excessive reduction in dissolved titanium concentrations is not to result by addition of higher proportions of seed). Hydrolysis temperatures above 150 give larger rutile diameters and higher hydrolysate yields as well as higher rutile contents.

A good compromise for Step 3 is hydrolysis at a titanium concentration of about 40 grams per liter, a chloride concentration of about 9.8 grams per liter, a seed level of about 4%, a temperature of 150 and a residence time at temperature of about 2 hours (under autogenous pressure).

EXAMPLE 1 (Steps 1 and 2) A seed dispersion was prepared from an aqueous solution of titanium chloride comprising about 15 g. Ti/ liter and about 24 g. Cl"/liter. The solution was brought to a temperature of about 85, maintained at that temperature for about 12 minutes and then cooled to room .temperature. Mild stirring was employed. The resulting product was a turbid, opalescent colloidal dispersion of hydrous titanium oxide particles.

(Step 3) The seed dispersion was mixed in a ratio of one volume to ten with a solution of TiCL; in hydrochloric acid, the solution containing about 37 g. Ti/liter and about 412 g. Cl-/liter. The resulting mixture thus containing about 35 g. (0.73 g. atom) Ti/liter and about 377 grams (10.6 gram ions) of chloride per liter (Cl/Ti atomic ratio=-14.5; wt. ratio Ti in seed to Ti in master solution=-0.04).

The mixture of seed and master solution was heated at 150 for two hours under autogenous pressure and with moderate stirring in a glass lined Pfaudler reactor. Filtration, acid washing and drying gave a 78% yield centration is at least 71/48 0.1=-0.15 gram per gram of titanium, or 2 0.1/48:-0.004 gram ions per liter. The effect of chloride concentration on growth rate is suflicient to afford some control. Higher concentrations result in lower growth rates. Thus, by going to a higher chloride level, the temperature can be increased somewhat without an increase in growth rate occurring. The chloride concentration also has an apparent effect on conversion. That is, the degree of hydrolysis achieved in a given finite reaction period varies inversely with the chloride concentration. For example, typical 2 hour hydrolyses carried out at 110 and at 150 showed higher final contents of dissolved titanium at higher chloride contents:

Final titanium concentration at Temperature g./liter g./1iter g./liter Dissolved titanium concentration and nucleant level The rate of hydrolysate precipitation is proportional to a power of the concentration, (Ti) with x being between 1.5 and 2. At concentrations above about 60 grams per liter the rate of accretion becomes excessive and control of particle shape is lost. In a batch hydrolysis, the effect of the dissolved titanium concentration is not independent of the number (and size) of the particles in the solid phase. The more particle surfaces presented for growth, the lower the rate of accretion (growth) per particle at a given rate of hydrolysate precipitation. Thus, better control can be maintained in the face of higher dissolved titanium levels it higher nucleant levels are employed.

If a continuous process is being operated, it is advantageous (speaking practically) to have a high nucleant level and the system can be controlled by the rate of addition of dissolved titanium. In this case, the steady state titanium concentration can be as low as 0.1 g. per liter if other conditions are such that the soluble titanium accretes (as hydrolysate) to the growing particles about as fast as it is being added to the system. Generally, however, a dissolved titanium concentration of from about 5 to 30 grams per liter is preferable.

The nucleant level (wt. ratio of titanium contents in nucleus crystals and solution) can range from 1 to 50 percent. The process is operable at a level of less than 1 percent but even at this level very inefficient use of reactor space results. The upper extreme is also imposed by practical considerations. Slurries of such high solids content are difficult to stir, circulate, filter, etc. The process can be operated at what, in effect, is a very high level by circulating the liquid phase through a bed of growing particles contained by filtration means. However, this is a less efficient mode of operation. Preferably, the amount of titanium present as nucleus crystals is 5 to 20 wt. percent of the dissolved titanium.

Phosphate, sulphate and bisulphate concentrations It has been found that the hydrolysis rate and rutile content of the hydrolysate are adversely efiected by multivalent anions such as phosphate, sulphate and bisulphate. The process can be operated at sulphate levels as high as 0.01 gram ions per liter but phosphate levels above 6 0.001 gram ions per liter are generally not operable. Preferably, the levels of phosphate and sulphate (or bisulphate) are an order of magnitude lower, i.e., are respectively less than 0.0001 and 0.001 gram ions per liter. Since titanium ores comprise quite appreciable amounts of acid leachable phosphate, titanium chloride solutions such as hydrochloric acid leaches of ilmenite must be essentially freed of phosphate (as by ion-exchange) in order to be suitable for the practice of the instant invention.

Residence time Growth ratios of about 2 can be achieved on nucleus crystals contacted with suitable titanium chloride solutions for as little as about 3 minutes. Under conditions of temperature, titanium concentration and chloride concentration previously indicated as not resulting in excessive growth rates, a growth ratio as high as 4 can be reached in as short a time as 12 minutes. However, a contact time of from /2 to 2 hours is preferred. Residence times of up to 50 hours or more are operable, particularly when nucleus crystals having aspect ratios of or more are employed and conditions are such that a slow growth rate prevails.

Preparation of suitable titanium chloride solutions Titanium chloride solutions for the practice of the present invention can be obtained in several ways. For example, anhydrous titanium tetrachloride can be reacted with ice according to U.S. Pat. 3,425,796 to provide a concentrated solution having a chloride to titanium ratio corresponding approximately to the formula TiOCI This can then be diluted with water and/or hydrochloric acid to the desired composition. Alternatively, titanium tetrachloride can be reacted with water if means are provided for dealing with the considerable heat and HCl evolution involved. The hydrolysis can be moderated if, for example, hydrochloric acid is employed as the water source material. Hydrochloric acid leaching of titanium ores, such as ilmenite, will provide titanium chloride solutions. However, these must be essentially freed of impurities detrimental to the formation or end use of the rutile particles of the invention. Such impurities are, for example, phosphate, and sufficient amounts of iron to discolor the product. Also, it will generally be necessary to reduce the chloride concentration of the leach, as by evaporation, for example.

Preparation of suitable nucleants Monocrystalline rutile hydrolysates prepared from iron free leaches of ilmenite ores according to the 484 patent (Long et al.) are suitable as nucleants for the practice of the instant invention. Since these crystals are of relatively low aspect ratio to start with, high growth ratios will necessarily result in a more significant change in shape for the product particle vs. the nucleus crystal. Consequently, a choice must be made between relatively low growth ratios or a reduced scope of utility of the product particles as pigments. Also, nucleants prepared according to said patent are less preferred because high hydrolysis temperatures (or calcining) must be resorted to in order to assure high contents of the requisite monocrystalline nucleus crystals.

Generally similar considerations apply to the similar rutile crystals made by the process of the 773 patent (Surls et al.), wherein a titanium chloride solution comprising from 6 to 11 gram ions of chloride per liter is hydrolyzed at 85-15 0 C. and the resulting hydrolysate is then treated at -300" C. with dilute hydrochloric acid (or with a less corrosive acid, such as nitric acid).

An alternative two step process to that of the 773 patent has also been discovered. An acidic aqueous titanium chloride solution containing from 0.5 to 4.5 gram ions of chloride per liter and from 10 to 60 grams of disof product, based on the total titanium charged to the reactor. By X-ray diffraction, this product consisted of brookite 53% and rutile (47%) having an average diameter in the 110 plane of 410 angstroms (0.04 micron) by line broadening. A representative sample of the unfractionated product was deposited on a colloidion slide surface by dipping the slide into a vial containing a mixture of product and hexane under supersonic vibration (to effect dispersion) and then allowing the hexane to evaporate. Electron photomicrographs of the slide were then prepared at magnifications of 13,000 and 50,000 and examined. The product particles were found to have a diameter range of from 0.01 to 0.1 micron, a length range of from 0.1 to 3.0 microns and an average length to diameter ratio of about 50 to 1 (some up to 60 to 1). The rutile particles were determined by electron difiraction (dark field technique) to be monocrystalline. The Reynolds tinting strength of the product was determined to be about 500, by the procedure commonly used for rating TiO pigments and described in the 484 patent.

(Step 4) A portion of the product slurry was filtered and the brookite fines were removed by redispersion and washing with 0.1 M HCl and filtering. The filter-cake (needle seed fraction) corresponded to 59% of the starting prodnot and consisted of 89% rutile of about 0.043 micron average diameter and about 11% of unseparated brookite fines by X-ray diffraction analysis. The needle seed fraction had a tinting strength of 1200, an average rutile diameter of 0.05 micron and an average rutile length of about 3.5 microns.

(Step 5; practice of the instant invention) A starting hydrolysis mixture was prepared by suspending 2.08 grams of the preceding TiO needle product (dry weight basis) in 200 ml. of a solution containing 7.3 g./l. soluble Ti, 6 g. ions/ l. chloride and no phosphate or sulphate. This mixture was introduced into a glass reaction flask and heated to 100 C. under atmospheric pressure. To the stirring mixture was continuously added a solution containing 80 g./1. soluble Ti and 6 g. ions/l. chloride, at a rate of 1 ml./min. A sample of the resulting slurry was removed at various time intervals. Each product was examined for light scattering efliciency by tinting strength measurements.

The eifect of chloride level on growth was examined by repeating the experiment at chloride levels of 5 and 7 g. ions/liter. The results of all three growth studies are given in Table I.

The progressive decrease in average 110 rutile diameter from the initial to final samples in each set will be noted. At first, the larger core needles are the only rutile crystallites present but as the forming particles grow, these are considerably outnumbered by the increasingly higher proportion of crystallites clustered around them. The growing bundle is no longer a single crystal and its overall diameter does not affect the determination.

The average diameters of the growing particles at 20, 30, and minutes (6 g. ions Cl"/liter set) were calculated by multiplying the square roots of the growth ratios by 0.05, the average diameter of the nuclei crystals. Dividing these into the average length (3.5 microns) of the nuclei crystals gave the respective average aspect ratios of the product particles. The values so found are given in the following Table Ia.

TABLE 1a.-PRODUCT PARTICLE PROPERTIES 1 G.R.= Growth ratio.

The preceding example not only constitutes a growth study but also illustrates a semi-continuous mode of operation of the instant process invention.

EXAMPLE 2 An acidic aqueous titanium chloride solution containing 35 g./ liter of titanium and 10 g. ions/liter of chloride was mixed with a colloidal Ti0 seed (4% level), heated to C. and maintained at that temperature for 2 hours. The resulting hydrolysate was filtered out, washed with water and stored as a wet cake under dilute (0.2 M) hydrochloric acid.

Nucleant samples were obtained as needed for the following experiments by redispersing the wet cake in the dilute acid, withdrawing a portion and filtering. In this manner peptizeable non-rutile fines were removed to provide a needle fraction as the filtrand from each sample. A number of hydrolyses were carried out with the diflFerent nucleant portions in sealed glass ampoules at difierent nucleant levels and under several conditions of temperature and chloride concentration, as shown in the following tables. Agitation was provided by rocking the bomb in which the ampoules were heated. The titanium chloride solutions used were derived from TiCL, and contained no detectable amounts of phosphate, sulphate or bisulphate.

TABLE I.-HYDROLYSIS STUDY Titanium Chloride Solution Added Continuously to 100 C. Hydrolyses Nucleated With Needle Monocrystal Fraction of Step 3 Hydrolysate Composition Product Ratio: g./l. Gram Ratlo Soluble Ti Growth composition, Avg.- soluble Ti ions per soluble S01. T1 Total ratio, percent rutile Sample time, to g. ions of liter Nucleant, Ti to Added, Found, pptd., prod., prod./ '1.S. diam. min. Cl-lliter Clg., Ti/l. nucleant g./1. g./l. g /1 g., Till. nucl. dry R 1 B 2 A B A;

R=ruti1e. 2 B=brookite. A=anatase. 4 By X-ray line broadening. 5 T.S.=tinting strength.

A chloride to titanium ratio of abou 0.67 g. ions/g. or about 0.017 atomic ratio;

Rutile size, A.

Dried product X-ray Tint Percent Percent Percent rutile brookite anatase Hydrolysis,

temp. Tube and No. time Nucleant G. Tl/l.

EXAMPLE 4 Deleterious efiect of phosphate anion on hydrolysis ate and hydrolysate tinting strength.

(A) A needle fraction was obtained as in Step 4 of Example 1 from a portion of the product slurry made in action was closely similar to that described in Example 1 and was employed as the nucleant in an attempted growth study. An amount of needle fraction which provided 5 grams of titanium (as TiO per liter of mixture was mixed with amounts of water, 0.36 molar aqueous phosphoric acid and 1?. molar aqueous hydrochloric acid such that the resulting slurry has a volume of 200 cc., contained 6 Step 3 of the same example. This needle fr gulate suspension to 12% of volume by (Step 3) The resulting seed concentrate was used at a level of 65 30% in a 2-hour hydrolysis under autogenous pressure (Step 4) When the hydrolysate was redispersed in dilute hydrochloric acid and filtered, 43% of it was recovered as a \l e h 000000000 n m ar a ma. mnwwmmmmw m 3 m my m m LLLLLLLLI 1 u n m n M m m Tm 6 1 t. a m m m m am I m m m a m 1 W rfiflr 000000000 n n d e e o wmm u n w s cH U A n m m 1 d .m 6 m m m mmwm m 0 .00? n n m n e I a n 0 n n m 9mm n 0 9 .1/|\ u n n P a us E n n n 6 aM d L 2495mm5o m m u H mm fi0 fi B a 1 11 m A G mam m m 1 t w P T [i l 1 613 5 3 Y d a e 6 7 1 v 1 1 1 1 1 1 1 1 1 Sib 1.1) HW MWM 67441 23128 5905563 94462 120302 fiownwi4nwfiififii m mj m m ufi md B a: 8 7 6 55 am 054 03 07 0055 $7 655 7 5 fiwd 3 2 Z5542w "m H m m m m n u S 6 e n n 0 r 7 m3 m2 9 m2 To m m0 m1 0 m0 g qrm m m WMWMW wfimmwmw %m% mm mum H .m n D.

m m 2 6 m m mm m m m m m "m w m m m mm m 0 dama e 1 1 5 1 0 00.00.0000 5 0 0 m m m m m m 0 m 4 5 5 6 u u a u n 8 .00am m Mami am 0 n mwfiwm ufieo ll l afi 1246 1m m a H W V Ma 10 e .1 C O1 6 t.w 8 T 00% e a 0 m we we 00000 00000 0000000 00000 0 .0 0000000000000000000 .m m m m hfi t 1 n a 6 6 6 6 6 7 7 7 7 7 8&00000 7 7 7 7 7 0&86606 77777777888888999999 t e V m 5 e a W .B we I B r l. t d h 0 m0 m wd a am .0008 .1 wwwww mmwwwwmw wwmwwww wmwwwww Mw%%%%% %%%%%N%%%%332%%%3322 d M g m m mf me .1. J1 11 3 w n m flaw a x C 1 me m0 0 I m mm mmm mw D I v I s I I a 0 mm BA. mm m v m Hm wwm m n am M wwawm e wt 70mm mt m m W d n wb e a c o & m n m W m w MW 0 M8 1 d & OOUHHTI m 6.16C W i P B 1 0 8: W R PSuhO .e. m .a ow %0m am won m ee E .m m ma v mm m %Wm mmnm %Wm 1w yr H e m 9 d d s 0 7 1 n w .00 eW m h 7 0o 7 7 f 1 1 a t e U 5 w s w m $.mmw M .1. 0 o e h e t r 01 on w 10 0 m a ma e mm i t W2 0 20 0 2i cm O oOO b dm n m 1 1 m 0P ammmzofi mmkZ & f. 1 I 23 5 7 012 e 6 1 a 30 1 0m unis? 0 10m umijsiismmnf 3 2?00 1. m s 0 m m m met 0 6 S l R e o e g a C U n O E 9 a S H 0 t. .l 6 L 1 n 2. C 6 u n m HW MH mM m Am s I L B C D m a a skfim m a U c u resulting coa centrifuging.

at 150 C. of a solution containing 40 grams of titanium and 10 gram ions of chloride per liter. The hydrolysate accounted for of the total titanium charged.

% rutile needle fraction having an average diameter of 290 angstroms (0.03 micron).

g. ions of chloride per liter and was 0.01 molar in phosphoric acid (H PO This was heated to 100 C. and to it was added 20 cc. of an acidic titanium chloride solution containing 100 grams of dissolved titanium and 6 gram ions of chloride per liter. The mixture was stirred 14 had a tinting strength of 1450 and showed a performance relative to a commercial anatase pigment of 130% in a paper opacification test (based on the TAPPP 425 m-60 standard procedure as follows).

at a temperature of -100 C. for 80 minutes and 50 Evflluanon of T102 i opaqu? filler for Paper cc. of the chloride solution was added at a rate of 1 cc. The 2 mp was dlspefsed 111 Water at P 8 using per minute. At ten minute intervals, samples of the reacall llllrasomc disperser and NaOH as Yequimd for P tion slurry were withdrawn. Growth ratios, tinting control gof p e was used to Prepare 0f strengths, and dissolved titanium concentrations were de- 10 pigment solids. Bleached Kraft softwood pu p was soaked termined as shown in the following Table IV: in water and refined in a laboratory size Valley Beater TABLE IV Ti, Dlss. Growth Init; soln. Sample Final Seed Ti Anal. Total ratio vol., add, out, vol., present, add., so Pptd solids, product] T.S., cc. cc. co. co. g. i/l. g.l T1, g./l Tl,g./l g. Ti/l. seed dry o 200 5.00 5.0 1.0 1,250 200 20 220 4.55 9.1 220 9 20 209 4. 37 12. 7 13. 3 Nil 4. 4 1. 0 1, 200 2 09 11 20 200 4.15 17. 0 1e. 9 Nil 4. 2 1. 0 1,200 200 10 15 195 a 95 21. o 20.9 Nil 4. 0 1. o 1, 150 195 11 15 191 3. 74 25. 2 25. 4 N11 3. 7 1. 0 1, 150 191 9 10 190 s. 57 28.6 27. s o. s 4. 4 1. 2 190 10 180 3. 57 28.6 22. 3 6.3 9. 9 2.8 180 10 170 3. 57 2s. 6 5. 2 23. 4 27. 0 7. 0 950 Essentially no growth occurred until about 70 cc. of to a Canadian Standard Freeness of 350. A sample of the chloride solution had been added, i.e., until the phosphate beaten pulp was diluted in a beaker and the appropriate concentration was appreciably less (due to samples revolume of pigment dispersion was added to obtain the demoved between additions) than 200/270x001 or sired filler level of 0, 4 or 6% based on the weight of 0.0074 mole/liter. The tinting strength of the product paper. The mixture was stirred and adjusted to pH 5.5 at a growth ratio of 7 was only 950, as compared to about and then flocculated by the addition of A1 (SO solu- 1500 at a comparable ratio in Example 1 (6 g. ions Cl/ tion at 40 lbs. alum per ton of pulp and Separan PG-2 liter). X-ray analysis of the solid phase from the 60 min- Solution at 0.5 lb. per ton. The entire fiocculated mixture ute sample (growth ratio 2.8) showed 81% rutile and was diluted further in water, divided into five equal por- 19% brookite with an average rutile diameter of 200 tions, and each was made into a sheet of paper measuring 8 angstroms. Thus, in comparison to the composition of inches by 8 inches and weighing about 2.5 g. dry. The the nucleant (see Example 1), as well as from the drop sheets were made by standard procedures using a Nobel & off in tinting strength with growth shown in the preced- Wood Hand Sheet machine. Five replicate sheets were preing table, it is evident that the hydrolysate which did form pared at each T10 filler level. In a similar fashion test in the instant experiment was not rutile. sheets were made using the reference or standard TiO (B) Three identical portions of an acidic titanium chlofiller, (Du Pont LW anatase). ride solution containing 30 grams of titanium per liter The paper sheets were conditioned overnight in an and 7 gram ions of chloride per liter were mixed (18% atmosphere of ca. 20 C., relative humidity and level) with essentially identical portions of a rutile needle then weighed. Three of each replicate set were chosen nucleant closely similar to that described in Example 2. 45 with weights closest to 2.48 g. and these were used to One mixture was made 0.01 molar in phosphoric acid and determine the opacity and brightness at each filler level. another 0.01 molar in sulfuric acid, while nothing was Reflectance measurements were made with a Bausch & added to the third mixture. After 2 hours at 150 C. Lomb Spectronic 505 refiectometer at 550 nm. which (sealed tubes), the solid phase in each mixture was sepahad been calibrated using the N.B.S. standard opaque rated, weighed and analyzed by X-ray. The results are 5 white plate. For each set 8 replicate determinations were given in Table IV-B: made of total reflectivity or brightness (R..) on a thick TABLE W43 pad of the paper. In a similar way 16 replicate measure- Pement Average ments were made of the opacity (R on single sheets Additive Tinting rutile of the paper backed by a black glass substrate of zero concentration strength Rutile Broolclte Anatase diam A- 5 reflectivity. The amount 0f TiO filler in each Set was 0 1,750 90 9 1 270 determined y ard gravimetric ashing procedures, 0.01MSO.-----. 1,050 65 26 9 280 The treatment of data was done by computer using 0.01MPOi-- s00 12 4 88 480 a program based on the Kubelka-Munk expressions of light scattering and absorption. These are discussed and (C) In tests similar to test B preceding, it is found 233E i a ggz g gg er 2 15 m :L fi g necessary to reduce the level of phosphate to 0.001 molar R vahies i 89 53 Wa z e g g an in order to get appreciable growth of rutile on the nucleus lazionshi s mpu e om t e needles and bisulphate @150 is found to be equivalent to sulfate ion as a poison. iv- 0-1) 1-.s9R., EXAMPLE 5 Where Utility for paper opacification 1/R.,, +R

2 A growth study was carried out at 100 C. (atmospheric pressure) using a needle nucleant essentially the same as that described in Example 2 with an aqueous solution 15 Y reflectanciof a $mg1e P of P p backed derived from anhydrous Ticl4 and Containing about 5 by a white substrate with a reflectivity of 89%. The final grams of titanium and about 7 g. ions of chloride per liter. The product particles present at a growth ratio of 15 Technical Association of the Pulp and Paper Industry.

expression of opacity and brightness of the paper is a contrast ratio known as Tappi Opacity The experimental results were corrected to a standard basis weight of 60 g./M. or, its equivalent, 2.48 g. per 8" x 8 sheet.

The relative opacifying power of the test sample to the standard was determnied by plotting the respective Tappi Opacity versus the filler content. From such a plot the amount of Ti0 required to attain T.O.:80 was 5.00% for the standard anatase and 3.85% for the test sample. Expressed as a ratio indicating relative performance the test sample was 130% of the standard under the given conditions.

Tests by the same procedure on higher tinting strength products made by the instant process show better than 130% performance relative to LW anatase.

If desired, the composite particles of the instant invention can be converted to rutile monocrystals having tinting strengths of up to 2100 or more by calcining in the conventional manner.

Physical/chemical characterization of product particles Composite pigment particles prepared according to the preceding examples are characterized by electron microscopy as to size and shape using the conventional sample preparation procedure for dry powder samples. A dispersion of the dry powder is prepared in distilled water using pH control and ultrasonic waves to insure adequate dispersion into the unit acting particle. A dispersion concentration of about 0.1 mg. of powder in one cubic centimeter of water is used. A portion of this dispersion is sprayed onto 200 A. thick carbon reinforced nitrocellulose films supported on 200 mesh electroformed copper screens. These specimens are observed with an electron microscope (Hitachi HUl 1A, for example) in the high resolution transmission position at 100 kv. ac celerating voltage. The magnification of the microscope is determined by photographing a 28,800 line/inch replica of an optical grating.

The actual characterization of the internal structure of the particle is also done by electron microscopy using ultra thin sectioning techniques to reveal the structure in cross section.

To accomplish this a 100-500 mg. sample of washed, salt-free filter cake of the product particles is suspended in 10 cc. of absolute ethyl alcohol to replace the water and mixed 10-15 minutes using ultrasonic techniques. Following this the particles are sedimented (as in a Sorvall GLC-l centrifuge at 4000 r.p.m.). The supernatant ethanol is decanted and the sedimented particles resuspended in fresh absolute ethanol. This suspension and sedimentation step is repeated until the particles are free of residual water. The alcohol is then replaced in a similar fashion with propylene oxide as a transitional solvent using two or three volume replacement washings. The particles are embedded in an epoxy resin using a graded series of resins in the transitional solvent propylene oxide, i.e., 90/10 (propylene oxide/epoxy resin) (75/25, 50/50, 25/75, 100%)(epoxy resin). The TiO particles are suspended in each resin component for 3045 minutes, centrifugally sedimented and the sediment transferred to the next higher resin concentration, repeating this procedure until the particles are brought into equilibrium with 100% epoxy resin. The impregnated crystal resin specimen is transferred to gelatin capsules and heat polymerized to yield a mechanically hard block suitable for ultra-thin sectioning. Using an ultramicrotome equipped with a diamond knife sections are prepared that measure 400-600 A. in thickness for study in the 16 transmission electron microscope. The electronmicroscopy is carried out with an electron microscope, such as the Hitachi HUI 1A, at kv. using a 60 wide angle tilting stage especially designed for crystallographic orientation studies. The instrument is calibrated with a 54,864 line/inch replica of a diffraction grating.

A sample of a typical growth, essentially identical to that of Example 2 (group C, tube No. 5), was characterized as follows. Bright field examination in a thin section revealed that the bulk cross section was circular and 1000 to 1800 angstroms in diameter but with fine structural detail. High magnification study revealed that these bulk particles were composed of two distinct sizes of crystals in a unique relationship to one another. The particles consisted of a central core or nucleus of one or more larger crystals measuring 250 to 500 angstroms (0.0250.05 micron) in diameter surrounded by densely packed irregular layers of much smaller crystallites measuring 0.002 to 0.02 micron in diameter. Dark field examination and electron diffraction in the electron microscope revealed that the larger nucleus crystals were rultile with the fiber axis lying along the [002] crystallographic direction, i.e., perpendicular to the plane of the micrograph. From the sharp bright-to-dark contrast reversal observed by tilting the specimen one to three degrees off the [002] orientation, the smaller crystallites were also identified as rutile and determined to have a predominant [002] fiber orientation.

Examination of a range of such growth products has shown that the interstitial space between the smaller crystallites comprising the bulk of the total particle is non-rutile in nature and its relative volume varies from 110% of the total particle volume depending upon the particular crystal preparation condition. When the specific product sample above described was examined the nucleus or seed crystal was found to represent 8.3% of the particle volume, the surrounding growth on this seed crystal consisted of smaller (SO-200 angstrom) rutile crystallites which accounted for 87.6% of the total particle volume with the interstitial non-rutile volume measuring approximately 4.1%.

Sections were prepared at approximately 45 to the fiber axis corresponding to the [302] crystallographic direction. This grazing angle section showed that the smaller crystallites were parallel With the fiber axis of the nucleus or seed crystal. The morphology of the small crystallites was therefore determined to be acicular. From this section study they were found to be monocrystalline and greater than 0.1 micron (1000 A.) in length.

Examinations of the range of composite particles made by the process of the instant invention shows that the rutile crystallites which accrete to the nucleus crystals range in length from 0.025 to 2 microns, in diameter from 0.005 to 0.05 micron and have aspect ratios of from 5 to 50.

We claim: 1. A titanium dioxide pigment particle consisting of one or more monocrystalline rutile nucleus crystals elongated on the c rutile axis, a plurality of similarly elongated rutile crystallites disposed around said nucleus crystals in a configuration generally parallel thereto and non-rutile interstitial titanium dioxide,

said particle having a length within the range of from 0.15 to 15 microns, an effective diameter of from 0.05 to 0.3 micron and an aspect ratio of from 3 to said nucleus crystals having lengths within the range of from 0.15 to 15 microns, diameters Within the range of from 0.01 to 0.1 micron, aspect ratios of from 5 to 200 and being generally parallel to each other when more than one is present in said pigment particle; said crystallites having lengths within the range of 0.025 to 2 microns, diameters within the range of 0.005 to 0.05 micron and aspect ratios of from 5 to 50;

and said interstitial titanium dioxide constituting from 1 to 10 volume percent of said pigment particle.

3,7 49,7 64 17 18 2. A particle as defined in claim 1 having a length of R ferences Cited at least one micron and in which said nucleus crystals have diameters within the range of 0.01 to 0.04 micron, lengths UNITED STATES PATENTS of at least 1 micron and aspect ratios of at least 25. 3,329,434 7/ 1967 8 et a1 423611 3. A particle as defined in claim 1 and having an etfec- 5 3,549,322 12/1970 Klfiln 6t 4 3 X tive diameter within the range of 0.08 to 0.12 micron. 3,703,357 11/1972 SW15 et a1 423511 4. A particle as defined in claim 2 and comprising not EDWARD STERN Primary Examiner more than 5 volume percent of non-rutile titanium dioxide.

5. A particle as defined in claim 3 and comprising not U.S. Cl. X.R. more than 5 volume percent of non-rutile titanium dioxide. 10 423-611; 106-300 

